I would have preferred a 400 f/4.0L IS it maybe would be a little more bulky, but would make it far more useful with a 1.4 or 2.0 tc
Go buy one, they have been available for many years. The 400mm f/4 DO is described as a "L" lens by Canon.
Nothing quite compares to the amount of dimension a super telephoto lens can bring to a photograph. All the lenses in the category are part of the L-series, incorporating superior lens construction and USM for quiet, high-speed autofocusing.